Elastic monofilms based on styrene block copolymers are frequently used in practice for elastic laminates from which elastic sections, such as diaper ears or elastic closure strips of diapers, in particular baby diapers, are formed.
For an elastic monofilm, it must be noted that the elastic monofilm, in particular when used on a disposable article, should be as thin as possible and therefore as economical as possible, while on the other hand must have sufficient load capacity and maximum tensile force.
Further conflicting requirements for such a monofilm are the requirement that, on the one hand, an elastic element formed from the monofilm should be evenly and, at the beginning of the elongation process, easily stretchable, while on the other hand the monofilm must have adequate tensile strength for the processing of the monofilm. In this way, excessive stretch or even tearing of the film must be prevented during handling.
The fact that, on the basis of a production direction of the film, the stretch behavior is different in the production direction (machine direction—MD) and perpendicular thereto in a transverse direction (CD), can be utilized to a certain extent. The monofilms are, in particular, processed into laminates and later into individual laminates in such a way that stretch occurs in the transverse direction (CD) in the finished product.
The known monofilms based on styrene block copolymers are frequently formed from symmetric triblock copolymers such as styrene-isoprene-styrene copolymer or styrene-butadiene-styrene copolymer.
An elastic monofilm of the type in question, comprising an asymmetric styrene-diene triblock copolymer at a content between 35 wt % and 90 wt % as a first polymer substance and comprising a second polymer substance, is known from EP 2 264 101 [U.S. Pat. No. 8,492,490]. The second polymer substance is also a styrene-diene triblock copolymer, that, however, has a symmetric or at least less asymmetric block structure.
Thus, the film is composed of styrene-diene triblock copolymer, wherein different triblock copolymers must be kept at hand and mixed with each other.